The invention relates to a device for providing heat energy for a gas-generating system in which hydrogen gas is generated from a gas which contains hydrocarbons.
For example, in the operation of fuel cells for mobile applications, hydrogen is generated from a raw gas containing hydrocarbons, especially methanol, by reformation of steam or partial oxidation. Heat energy is required to heat the system for a cold start, for evaporation and superheating of water and methanol, and for covering the energy requirement of endothermal steam reformation.
In this technology, the energy for covering the heat requirement is provided for example by burning hydrogen and/or methanol with an open flame. However, such systems exhibit poor partial load behavior because of the lack of flame stability. In addition, because of the high combustion temperatures, heat oxides of nitrogen are formed, which is especially undesirable for mobile applications.
In addition, heat-generating systems based on catalytic combustion of gases containing hydrocarbons are known, in which the temperatures are kept significantly below 1000.degree. C. during combustion so that formation of thermal oxides of nitrogen is prevented. U.S. Pat. No. 4,909,808, for example, discloses such a device in which a fuel/air mixture is supplied to a combustion chamber to provide heat energy for steam reformation, and is oxidized therein catalytically.
In such devices, the required heat energy can be generated either directly in the consuming component or in a central component, with the heat being supplied in the second case by a heat-conducting medium. An example of the first embodiment is known from U.S. Pat. No. 5,180,561. In this case, hydrogen is generated from methane by steam reformation in a plate reformer, with reaction chambers and catalytic combustion chambers alternating in the plate reformer. An example of the second embodiment is known from JP 4-164802 A. In this case, hydrogen is produced in a reactor by steam reformation of methanol, with the reactor being heated indirectly by a hot gas stream generated in a central catalytic combustion chamber. In order to ensure a rapid starting process, this Japanese document provides that, on starting the device, a mixture of methanol and preheated air is evaporated and supplied to the combustion chamber and a switch is made to a combustion gas with a higher ignition temperature only after a higher temperature has been reached.
The goal of the invention is to improve a device of the generic type described above for providing heat energy for a gas-generating system in terms of starting behavior and exhaust emissions, as well as providing a method for starting this device.
The device according to the invention has the advantage that cold-starting behavior is further improved and emission of unburned fuel can be prevented even in the partial load range. The provision of an additional component for cold-starting with an integrated heating device ensures rapid heating of the entire system. By using a residual gas component, especially in the cold-starting phase and in the partial load range, complete reaction of the fuel is ensured so that the undesired escape of unburned fuel is avoided. In the main combustion chamber, which is advantageously designed as a filter press, a heat exchanger is integrated to permit a compact design and good heat transfer.
By dividing the entire system into three components, with the cold-start component being designed for rapid response, the central component for high power density, and the residual gas component for complete reaction, a total system with rapid cold-start behavior, low exhaust emissions in all operating ranges, high maximum fuel concentration, and hence high load injection, and high dynamics during load changes is provided. In addition, the system can be readily regulated by adding fuel.
The claimed central component has a simplified design by virtue of the common supply of fuel and air. The possibility of adjusting a temperature profile deliberately over the length of the main combustion chamber makes it possible significantly to improve the efficiency, since the device can be operated with a high fuel concentration, high power level per combustion chamber, and a good distribution of the reaction over the entire length.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.